Intelligence and Miscellaneous Articles. 239 



duced by a current of strong tension, at the contact of the positive 

 or negative electrode with the sides of a glass or rock-crystal vessel 

 moistened with a solution of sea-salt, I observed that the glass or 

 crystal was powerfully attacked at the points touched by the elec- 

 trode, and that the concentric luminous rings formed around re- 

 mained sometimes engraved at the surface of the glass of the volta- 

 meter. On employing nitrate of potash as the saline solution, 

 much less electric force was required, than with chloride of sodium, 

 to produce these effects. 



These observations led me to apply the electric current to en- 

 grave on glass or rock-crystal. The surface of a plate of glass or 

 crystal is covered with a concentrated solution of nitrate of potass 

 by simply pouring the liquid on the plate placed horizontally on a 

 table or in a shallow basin. Next, a horizontal platinum wire, con- 

 nected with the poles of a secondary battery of from 50 to 60 ele- 

 ments, is immersed in the layer of liquid which covers the glass, 

 aloug the edges of the plate ; then, holding in his hand the other 

 electrode, consisting of a platinum wire enclosed, except at its ex- 

 tremity, in an insulating sheath, the operator touches the glass, 

 covered with the thin layer of saline solution, at the points where 

 he wishes to engrave characters or a design. 



A luminous trail is produced wherever the electrode touches ; 

 and whatever the rapidity with which one writes or draws, the 

 strokes made are neatly engraved on the glass*. If the writing or 

 drawing be done slowly, the strokes will be deeply engraved ; their 

 breadth will depend on the diameter of the wire serving as elec- 

 trode ; if it is pointed, the strokes can be made extremely fine. 



The engraving can be executed with either of the electrodes ; but 

 a less-powerful current is required for engraving with the negative. 



Although I have obtained these results by using secondary bat- 

 teries, it is clear that, for continuous work, any other source of 

 electricity can be employed in preference, if the quantity and ten- 

 sion be sufficient — either a Bunsen pile of a sufficient number of 

 elements, or a Gramme machine, or even a magneto-electric machine 

 with currents alternately positive and negative. — Annates de Chimie 

 et de Physique, Jan. 1878, tome xiii. pp. 143, 144. 



ON THE PHOTOMETRIC COMPARISON OF LIGHT OF DIFFERENT 

 COLOURS. BY PROF. 0. N. ROOD, OF COLUMBIA COLLEGE, U.S.A. 

 The comparison of the intensities of light of different colours 

 has long been considered one of the most difficult of photometric 

 problems ; but by the use of very simple means I have recently 

 made a series of measurements of this character which may not be 

 without interest to those whoss studies lie in this direction. The 

 luminosity of cardboard painted with vermilion was, for example, 

 measured as follows : — A circular disk of the vermilion cardboard 

 was attached to the axis of a rotation-apparatus, smaller circular 



* There is so often occasion to write or to mark lines on glass in labo- 

 ratories, that this process will there find frequent applications. 



